Prop Pitting, Cavitation and Maintaining Plane

[RJ17]

I have a 2011 Evinrude E-Tec 130 (that I absolutely love!) on an 18' Winner Center Console (with a full transom). It's a long story but I had to repower the boat in 2010 when the original engine bracket failed. The boat will move at about 33-35 MPH at WOT (about 5200 RPM) and likes to cruise at 22-24MPH (measured via GPS) at about 3200 RPM.

1st issue - I have the original aluminum prop (sorry, don't know the pitch or diameter) and it shows the normal wear and tear of an new/inexperienced boater. The paint is worn from the outside edge of all 3 blades (as well as the engine skeg), funny how sand and bottom do that! There are a few nicks & dings on the edges but nothing major. However, the front (bow facing side) of each blade has scarring and pitting at the point where the blade joins the hub. I believe this condition is due to cavitation and the resulting 'boiling water' and air bubbles 'burning' the aluminum.

Question 1 - Will a different prop (not stainless vs. aluminum) with a different pitch (or even diameter) resolve/reduce the cavitation and pitting issue? I realize that some of the bubbles may be the result of hull imperfections or height/positioning of the prop and the resulting stream of air bubbles that hit the blade and get compressed into the space near the hub,

2nd issue - My boat 'likes' to travel at speeds of 0-10 MPH and 20+ MPH. I can get up on plane in about 6-8 seconds and can maintain plane at about 20 MPH and 3100-3200 MPH. All well and good until there is a bit of a chop. As a result of the new bracket and engine the boat is very stern heavy. Even with the engine trimmed as are as possible, (tucked in as I like to describe it) the boat bounces pretty good in a 2+ foot sea. I know hull design and total weight are key factors in this equation but I do not like 'slamming' into the troughs at 20MPH. If I back off the throttle even a little I lose RPMs until I'm close to 2000 and the boat is moving at 10 - 12 MPH with the bow way up.

Question 2 - Can a different prop change the speed at which the boat can get on, and stay on plane? I'd certainly be willing to sacrifice some top end speed to be able to plane at 15-18MPH.

Thanks for any and all help

 

[GoldDuster360]

I have a 16-1/2ft Sylvan Super Sportster modified V that exhibits the same ride characteristics you describe. It too has a prop ventilation/caviation problem when trimmed where I think it needs to be. I just bought a stainless prop with more aggressive cupping than my aluminum one for it and have not tested it yet. If you want the pro's to answer your questions you may want to include hull type/Deadrise, weight of boat when loaded, prop pitch and diameter. That being said, if you want the boat to plain at a lower speed you may want to consider a 4 bladed prop. Stainless steel props generally get more aggressive cupping which should reduce ventilation and maybe cavitation. If you pull your prop there should either be a part number or the pitch/diameter given on the inside of the hub part that rests on the thrust washer.

 

[steelespike]

As I understand; cavitation burn is an indication a prop is working really well.
I have 4 identical props 11" and one 12" These date back to 1965 with the original white prop.
These all have cavitation burn the original it is extensive about 1/3 of each blade.
Usually a 4 blade is suggested to help improve hole shot and keep the boat on plane.
Something like Smart tabs can also help. You might consider Turning Point or Solas props In a 4 blade if they are available.

 

[RJ17]

Just to update - in the fall my boatyard tried several things to try and solve item #2, the planing issue. Their solution was to bolt on a custom "whale tail" (I'll add a picture later) to the motor. Frankly, I noticed little to no difference in the performance of the boat.

As far as the prop is concerned, it is my boatyard's opinion that changing the diameter or pitch will do little to change the cavitation/scarring issue. The cavitation and resulting 'boiling' are due to the hull design and relation of the prop to the waterline, hull, etc. (In another post I am trying to find out what the deadrise of my hull is at the transom.) They did however, recommend replacing the prop saying that the amount of scarring (after 5 seasons) was enough that the blades were weakened and the risk of snapping one off was great.

 

[Texasmark]

I only had cavitation burns on one prop in 50+ years of boating. It was an original OEM, white, aluminum prop on a NEW 1971 Chrysler 85 hp. The prop had a unique reverse twist on the blade leading edge, just after the departure from the barrel where the blades left the barrel (the blade root) at right angles, and it made a reverse swirl back forward for about half an inch then in a sweeping arc made the normal around the blade like conventional (non raked blade shape) props.

The boiling took place right inboard of that reversal and was about the size of a nickel coin after just a few outings. The boat was a 1971 16' Chrysler Sport Fury (very light and shallow depth tri-hull) and had a smooth, pretty much flat bottom across the center 3/4ths of the transom. The prop had no rake or cupping.

Based upon all my other boats, engines, and props, in many variations and sizes thru 18', I put the culprit as the prop design.

However, I am not a professional mechanic, designer, or product test tech for a marine parts supplier; in short, no authority on the subject. Just telling it as it happened to me as to cavitation burning and it's cause. I have read where prop dings and such are encountered, cavitation can result. However, just a couple of days ago, on here we were furnished with a picture of cavitation burn across ⅔ the length of the blade, slightly inboard of the tip with no apparent deformation to entice it.

I can see how a "hard working" prop can cause it as it/they (the prop's blades) move through the water faster than the water can follow them. This "hole in the water" on the back side of the blade just in from the tip,is a low pressure area and as such reduces the boiling point of the ambient water allowing it to boil resulting in oxidation/etching of the immediate area. Surely you have seen videos of props running in the water, usually at lower speeds, and saw the little bubbles following the blade tips.

 

[jimmbo]

Cavitation burn is not an indication of how well a prop is performing. It is an indicator of a design problem, or an interruption of clean water flow. The only time I ever experienced it was on a 1965 90 hp evinrude, That engine, like most OMC V4s1960 - 1968) had a very limited selection of props, six of them. 4 of the props had a hub that more or less matched the size of the gear housing, the other two, had a hub that was much smaller. We had the 10 X 9 1/4 version and after a few hours the paint started to disappear on the low pressure side blades near the hub right in line with the edge of the gear housing. Over a 3 yr period the erosion included a noticeable amount of aluminium disappearing.

To the OP. A boat has to be going so fast to plane. trim tabs, engine trim ,and weight distribution will affect what that speed is. Propellers on the other hand have very little effect on a boat planning speed. Oh some will rant and rave about props known as 'stern lifters' and others that are 'bow lifters'. Those characteristics really don't appear until well past 'hump' speed

 

[Texasmark]

Cavitation burn is not an indication of how well a prop is performing. It is an indicator of a design problem, or an interruption of clean water flow. The only time I ever experienced it was on a 1965 90 hp evinrude, That engine, like most OMC V4s1960 - 1968) had a very limited selection of props, six of them. 4 of the props had a hub that more or less matched the size of the gear housing, the other two, had a hub that was much smaller. We had the 10 X 9 1/4 version and after a few hours the paint started to disappear on the low pressure side blades near the hub right in line with the edge of the gear housing. Over a 3 yr period the erosion included a noticeable amount of aluminium disappearing.

To the OP. A boat has to be going so fast to plane. trim tabs, engine trim ,and weight distribution will affect what that speed is. Propellers on the other hand have very little effect on a boat planning speed. Oh some will rant and rave about props known as 'stern lifters' and others that are 'bow lifters'. Those characteristics really don't appear until well past 'hump' speed

I remember those OMC days , gearbox dimensions and prop hubs, when they had the conventional lower unit with the gearbox that broke open from the bottom half and then they came out with the bullet housing on their upper end offerings....Evinrude Lark for one, and then the giant Fat 50 gearbox. I was so happy that they upgraded their lower units in the late 60's I bought a 125 in '72 and chunked the Chrysler.

Wasn't the hub without a rubber insert and only had the "shear" (drive some call it on here) pin the small diameter hub? Easy how to see that the offset on those engines as you mentioned were cavitation when, not if.

 

[jimmbo]

The Fat-Fifty had a gear case like their 25, 30, 35, and 40 mechanical shift, just bigger, and a similar gear ratio. The bullet shape was introduced in 1960 on the 75hp but was later used on the 60, 65, 80, 85, and 90, both mechanical and electric shift models, and had a weird gear ratio of 23:20. The 40 electric shift also used the bullet case. All propellers had a rubber hub, and all drive pins were stainless so shearing them was difficult to do. In 1968 the new modern gear case was introduced on the 55hp, and extended to the rest of the higher HP in 1969 when OMC brought out totally redesigned V4s.

 

[Texasmark]

My '72 had most of the improvements of the Evinrude 55 hp Triumph except for the loop charging which at the time was limited to triples due to the exhaust scavenging reflected pulse. However, the V4 did have everything else and fuel efficiency was much better (not as good as loopers, but much better) than the predecessors and as I recall the fuel-oil mix was reduced....50:1? Don't remember.

 

[jimmbo]

To the OP(Original Poster)
A stainless prop will not show Cavitation Burn as quickly or as severely as on an AL prop. As I said in my earlier post Trim Tabs will lower the planning speed of the hull, perhaps you should consider them. Using Tabs to trim the boat allows the engine to be trimmed at a more efficient thrust angle.

To Tex: The main reason Loop-charging wasn't brought into the V4s till the mid 1980s was simply casting techniques. 3 cylinder engines are much easier to exhaust tune than 2 cylinder or 4 cylinder. OMC adopted the 50:1 ratio across the entire line in 1964.